This invention relates to ballscrew actuators and more particularly to an improved apparatus and means for reducing backlash and deadband positional inaccuracies in such actuators.
Conventional ball screw actuators commonly include a leadscrew having a helical groove along the length, and a follower with a mating groove that cooperates with the groove in the leadscrew to form a course or race for ball bearings. This course usually contains a plurality of ball bearings which operate in rolling contact with both the leadscrew groove and follower groove as one is relatively rotated with respect to the other. The ball bearings are thus positionally displaced along the course as the leadscrew and follower relatively rotate, and such course is therefore commonly closed by a channel that is coupled from end to end of the course through the follower. In this way, the balls may be continuously circulated from end to end through the course as relative rotation progresses.
One common difficulty encountered with conventional ball screw actuators of this type is that the mating grooves in both the leadscrew and in the follower may not form a course of accurate dimension throughout to accommodate the balls therein in rolling contact simultaneously with both the leadscrew groove and follower groove. If the balls are of diameter slightly smaller than the dimensions of the course, then there may be axial and lateral `play` or backlash between the leadscrew and follower, and this contributes to positional inaccuracies for a specific relative angular rotation of the two elements. Various schemes are known for eliminating such backlash, including slightly altering the pitch of the leadscrew or follower groove relative to the other so that some balls within the course are in compressional loading toward the center of the follower, while others of the balls within the course are in compressional loading toward the ends of the follower. A similar scheme relies upon another follower rigidly linked to the one follower with a fixed angular displacement between them similar to locking two nuts together on a common bolt. Still another scheme relies upon keyed, axially slideable follower end sections that are spring-biased apart.
These schemes for eliminating positional inaccuracies in ball screw actuators have not been acceptable in certain precision applications due to excessive torque requirements, added weight and inertia of two followers, and the like.